Re-configurable SLMs based on liquid crystal (and other types of) devices are widely used for controlling and manipulating optical beams. In diffractive mode they may be used for three dimensional (3D) imaging [BROWN, C V and STANLEY, M, UK Patent Application GB2330471, Production of Moving Images for Holography] and for routing optical signals in telecommunications networks
The SLM modulates the complex amplitude of an incoming wave front (i.e. changes its phase and/or amplitude), which causes it to propagate in the desired manner. The SLM generally comprises a liquid crystal panel containing a number of individually addressed pixels, onto which a diffraction pattern or Computer Generated Hologram (CGH) is written [CAMERON, C D et al, SPIE Conference on Critical Technologies for the Future of Computing (San Diego, USA), July-August 2000, Computational Challenges of Emerging Novel True 3D Holographic Displays].
CGH 3D display systems typically use a computer to generate and/or store electronic copies of the hologram. This hologram is then replayed on an SLM which is switched to modulate (in transmission or reflection) light from a source which then passes through suitable replay optics, thereby providing a visible three-dimensional image to observers.
In one known system used in the production of three dimensional (3D) images, a single EASLM is addressed to produce successive different images which are imaged sequentially onto an OASLM arranged in a matrix of segments which forms a complete display. Once all the component images have been written to the OASLM a complete image or pattern can be presented to an observer, e.g. by illumination of the whole OASLM matrix by laser read light. This is described in U.S. Pat. No. 6,437,919, WO-GB98/03097, GB2330471, and has been described as Active Tiling™. This system relies on high speed switching in the EASLM and bistability in the OASLM material to retain the switched image whilst the read light is applied, to give a flicker free display.
Typically a SLM includes a layer of liquid crystal material arranged between two electrode-bearing walls to form a liquid crystal cell. The material is switched by application of an electric field to the liquid crystal material, e.g. by electrical waveforms applied to the electrodes.
A typical EASLM comprises a liquid crystal cell formed by two walls enclosing a layer of nematic or smectic liquid crystal material. Transparent electrode structures are formed as strips of row electrodes on one wall and strips of column electrodes on the other wall. Electrode intersections define pixels where the optical state of the liquid crystal material is switched by application of an electric voltage to appropriate row and column electrodes. The electrodes receive electrical signals from driver circuits controlled by a display controller. One known smectic EASLM uses an integrated circuit backplane, and DC balance is achieved by addressing to form a positive image followed by addressing to form the inverse, i.e. a negative image.
A typical OASLM is basically similar to the EASLM but includes a layer of a photosensitive material between electrodes on one wall and a bistable ferroelectric liquid crystal material. In some examples the electrodes are segmented so that electrical contact is made separately to each segment; in this way an image may be applied to more than one segment (commonly all segments) but a voltage only applied to one segment to effect latching of the image only at that one segment. The OASLM is addressed by application of a voltage to the electrodes and simultaneous application of light to selected parts of the photosensitive material. This combination switches the liquid crystal material at illuminated parts whilst other non-illuminated parts remain unswitched. A display is viewable from the side of the OASLM remote from the photosensitive layer.
A disadvantage of bistable ferro electric devices is their low diffraction efficiency which results in a low level of image brightness.